Concurrency

Concurrency model is one of the most important aspects of an Embedded Operating System. Applications for embedded micro-controllers require operating with multiple sources of events at one time. Furthermore an embedded system should be in a power-saving mode as often and as long as possible. Drone's goal is to make writing highly concurrent and power-efficient applications easy and correct.

First, let's see how conventional Embedded Operating Systems work. They allow you to create tasks that are running in parallel, each with its own stack:

However this is not how hardware is actually designed. In fact, processors can only execute a single task at a time. What conventional Operating Systems actually do, is that they are rapidly switching between tasks, to make them appear to be running in parallel:

That concurrency model, while having clear advantages for desktop and server operating systems, incurs noticeable overhead for embedded real-time systems. Also to protect from stack overflow errors it should be running on a processor with built-in Memory Management/Protection Unit, which is not the case for STM32F103.

Contrarily, modern hardware evolves in the direction of more elaborate interrupt controllers. For example, Nested Vectored Interrupt Controller, or NVIC, which can be found in each Cortex-M processor. It implements many hardware optimizations to reduce scheduling costs, such as late-arriving or tail-chaining. Drone OS utilizes such interrupt controllers to build strictly prioritized fully preemptive scheduling:

Only a task with a higher priority can preempt another task. And a task must completely relinquish the stack before completing or pausing to wait for an event or a resource. This allows Drone OS to use a single stack for all program tasks. This single stack is also protected from stack overflow errors by placing it at the border of the RAM.

So how Drone achieves such stack usage for tasks? Mainly by using Rust's async/await or generators syntax, which translate to state machines. The task state, which needs to be saved between resumption points, is stored much more compactly on the heap.

As an option Drone also implements conventional stateful tasks. Using such tasks one can integrate an existing blocking code with a Drone application, by allocating a separate stack. To use this feature safely, the processor must have an MMU/MPU. Otherwise creating such task is unsafe, because the safety from stack overflow couldn't be guaranteed.